Field of the Invention
The present invention relates to a conductive polymer composite, in particular, a conductive polymer composite useful for solid capacitors. The present invention also relates to a method for preparing the conductive polymer composite and to a solid capacitor using the conductive polymer composite.
Description of the Related Art
Capacitors are a type of electronic elements that are widely used in various electronic products. With advancement in technology development, electronic products are being developed in the direction of miniaturization and light weight, and the capacitors used in electronic products are required to be miniaturized and have a high capacitance and low impedance when being used at a high frequency.
Capacitors may be classified into conventional liquid capacitors and newly developed solid capacitors. In the electrolyte of early-stage aluminum liquid capacitor, a liquid electrolyte is used as a charge transfer substance. The main components of the liquid electrolyte include a high-boiling point alcohol, an ionic liquid, boric acid, phosphoric acid, an organic carboxylic acid, an ammonium salt, a high-polarity organic solvent, and a small amount of water. The components not only serve as charge transfer substances, but also have the function of patching a dielectric layer of aluminum oxide on an aluminum foil. If the internal aluminum metal is exposed due to defects on the dielectric layer of aluminum oxide, during the charge and discharge process of the capacitor, the electrolyte may react with the exposed aluminum metal and aluminum oxide is generated, thus achieving the patching function. However, although the conventional aluminum liquid capacitor can meet the requirement of high capacitance at a low cost, as the electrolyte used is a liquid, it has the disadvantages of low conductivity and poor high temperature resistance; moreover, in the process of aluminum oxide generation, hydrogen is also generated, and if excessive hydrogen is accumulated in the capacitor, capacitor rupture can easily occur, which will damage the electronic product. Although a hydrogen absorbing agent may be added to the liquid electrolyte to reduce the risk of capacity rupture, the problem is not eliminated. Moreover, although conventional liquid capacitors have higher capacitance, their applications are limited due to exhibiting a higher equivalent series resistance (ESR).
Accordingly, a new generation of solid capacitor is developed, in which the liquid electrolyte is directly replaced by a solid electrolyte. Conductive polymer has been developed as one kind of solid electrolytes. Anions of an oxidant are blended in the structure of the polymer as a dopant and holes are formed, so that the polymer has conductivity. Compared with the liquid electrolyte or a solid semiconductor complex salt such as tetracyanoquinodimethane (TCNQ) composite salt and inorganic semiconductor MnO2 used in conventional electrolyte capacitor, the conductive polymer has a higher conductivity and a suitable high-temperature insulation property, so the conductive polymer has propelled the development of the trend of using solid electrolyte in current electrolytic capacitors.
In addition to having long service life that is 6 times longer than that of a common capacitor, the solid capacitor has improved stability and its capacitance is not easily influenced by an ambient temperature and humidity in use. Additionally, the solid capacitor has the advantage of a low ESR, a low capacitance variation rate, an excellent frequency response (high frequency resistance), a high temperature resistance, and a high current resistance, and the problem of leakage and explosion is eliminated.
Jesse S. Shaffer et al disclose a method of using a conductive polymer in an electrolyte of an electrolytic capacitor for the first time in U.S. Pat. No. 4,609,971. The method includes immersing an anode aluminum foil of a capacitor in a mixture solution formed by a conductive polymer polyaniline powder and a dopant LiClO4, and then removing the solvent on the aluminum foil. Due to its excessively high molecular weight, polyaniline cannot permeate into micropores of the anode foil, so the impregnation rate of the capacitor obtained through this method is poor, and the impedance is high. Then, in order to enable the polymer to easily permeate into the micropores of the anode foil, Gerhard Hellwig et al disclose a chemical oxidation polymerization method of using a conductive polymer as an electrolyte of a capacitor in U.S. Pat. No. 4,803,596. The method includes respectively immersing a capacitor anode foil in a solution of a conductive polymer monomer and an oxidant, and polymerizing the conductive polymer monomer at a suitable condition, in which the conductive polymer electrolyte is accumulated to a sufficient thickness through multiple immersions. Thereafter, Friedrich Jonas et al of the Bayer Corporation in Germany disclose a method of manufacturing an aluminum solid capacitor with poly-3,4-ethylenedioxythiophene (PEDOT) as an electrolyte by using a monomer 3,4-ethylenedioxythiophene (EDOT) in combination with an oxidant iron (III) p-toluenesulfonate for the first time in U.S. Pat. No. 4,910,645. Moreover, it has been found that 3,4-ethylenedithiathiophene (EDTT) which is structurally related to EDOT can be converted to electroactive polymer (see Lambertus Groenendaal et. al, Adv. Mater. 2000, 12, No. 7).
The conductive polymer PEDOT has the advantages of a high heat resistance, a high conductivity, a high charge transfer velocity, being non-toxic, a long service life, and no occurrence of capacitor explosion when being applied in a capacitor. In industry, PEDOT is directly produced from the polymerization reaction of the monomer EDOT with iron p-toluenesulfonate in a capacitor. Such production involves an in situ reaction and can be classified into one-part method, two-part method, and multi-part method according to the immersing manners. One-part method includes immersing a capacitor element in a mixture solution of EDOT and iron p-toluenesulfonate, and conducting the polymerization with heat. Two-part method includes separately immersing a capacitor element with EDOT and iron p-toluenesulfonate, and then conducting the polymerization with heat. Nevertheless, for one-part method, the processing parameters should be carefully controlled so as to avoid the polymerization of EDOT before the immersion. As for the two-part method, the problem associated with solution contamination is easy to occur.
In addition, the PEDOT on the aluminum foil surface or pores that is polymerized through an in situ reaction mostly has a powder structure with a lower polymerization degree, and the physical properties of the powder structure are poor, so the powder structure cannot be easily adhered on the aluminum foil surface or pores as it is more likely to fall off from the surface or pores, which results in a limited withstand voltage and disallow the solid capacitor to exhibit a voltage of 16 V or higher.
To solve the above-mentioned problem, Stephan Kirchmeyer et al proposed synthesizing a conductive polymer by an ex situ polymerization reaction (J. Mater. Chem. 2005, 15, 2077-2088). Nevertheless, the polymer obtained from an ex situ polymerization reaction normally has the drawbacks of composing of less repeating units (composing of about 6 to 18 repeating units) and exhibiting a lower polymerization degree (exhibiting a weight average molecular weight of approximately less than 2500). Such a lower polymerization degree polymer cannot be utilized in a working environment requiring a high voltage.
Accordingly, the industry calls for the development of a conductive polymer with a higher polymerization degree and smaller particle size distribution for being applied in solid capacitors that can withstand a higher voltage and have good stability, so as to be useful in current electronic products that require to be miniaturized and have a high capacitance, high-temperature resistance and low impedance when being used at a high frequency.